Generator of periodically varying electric currents



Jan. 27, 1948. P. K. CHATTERJEA ET AL 2,434,907

GENERATOR OF PERIODICALLY VARYING ELECTRIC CURRENTS Filed June 22, 1943 2 Sheets-Sheet 1 f/G. A

I gm 5,176 1 n F l W 2 a INVENTOR.,

PRAFULLA K. CHATTERJ'EA BY AND LESLIE W. HOUGHTON ATTOR/Vf) 2 Sheets-Sheet 2 P. K. CHATTERJEA ETAL Filed June 22, 1945 77/ FIG. 3.

Jan. 27, 1948.

GENERATOR 0F PERIODICALLY VARYING ELECTRIC CURRENTS Patented Jan. 27, 1948 GENERATOR OF PERIODICALLY 'VARYING ELECTRIC CURRENTS .Prafulla I'Kumar Chatteriea and Leslie Wilfred Houghion,

London,

England,

assignors to "Standard Telephones and Cables Limited, London, 1 England, a British company Application June22, 1943,-Serial N0.'491,770 In GreatBritain July 21, 1942 7 Claims. 1

The present invention relates to generators of electric oscillations of special wav forms and makes use of the properties of thermally sensitive resistance elements known as thermistors for obtaining the waves.

Thermistors have been in use for some years and are characterized by a temperature coefiicient of resistance which may be either positive or negative and which is -moreover many times the corresponding coeflicient for a pure metal such as copper. This property renders thermistors part cularly suitable for a variety of special application in electric circuits.

Various difierent materials are available for the resistance element of a thermistor, these various materials having different properties in other respects; as one example, a resistance mater al'having a high negative temperature coefiicient-of resistance comprises amixture otmanganese oxide and nickel oxide, with or without the addition of certain othermetallic oxides, the mixture being suitably heat treated.

Thermistors have been employed. in two different forms: (a) known as a directly heated thermistor and comprising a resistance element of the thermally sensitive resistance material provided with suitable lead-out conductors or terminals, and (1)) known as an indirectly heated thermistor compris ng the element (a) provided in addition with a heating coil electrically insulated from the element. ,A directly heated thermistor is primarily intended to be controlled by the current which flows through it and which varies the temperature and also the resistance accordingly. Such a thermistor will also be affected by-the temperature. of its surroundings and may therefore be used for thermostatic control and like purposeswith or without direct heating by the current'fiowing through it. ;An indirectly heated thermistor is chiefly designed to be heated by a controlling current wh ch flows through the heating coil and which will usually, but not necessarily, be diiferent from the current which flows through the resistance element, but'this type of thermistor may also be subjected to either or both of the types of control applicabl to a directly heated thermistor.

More detailed information on the properties of ,thermistors will be found in an article by G. L. Pearson in the .Bell Laboratories Record Dec. 194i), page 106.

Regularly repeated oscillations I having special types of wave form are required forvariouspurposes. In particular, saw-tooth wavesare commonly used in television systems for synchronization purposes, and rectangular waves are used for multivibrator circuits. Such special waves aregenerally obtained in relaxation oscillator circuits employing gas discharge valves, or equivalent hard valve combinations. Such circuits are frequently complicated and require various power suppliesfor the valves, and moreover are often difficult to operate satisfactorily at very low frequencies.

The present specification describes simple circuits employing directly or indirectly heated thermistors for generating various types of wave form, such circuits not including any valves.

According to the invention'there is provided a generator of periodically varying electric currents comprising two ormore thermistors, and an electromagnetic switch having an operated position for each thermistor in which position it connects the corresponding thermistor to a source of heating current, means being provided to enable the thermistor to vary a current for operating the switch during the heating period so that when the operating current reaches a predetermined value the switch moves to another operated position.

Theinvention will be more clearly understood. from .the following detailed description of embodiments with reference to the accompanying drawing inwhich:

Fig. 1 shows a schematic circuit diagram of a generator of waves of the saw-tooth type and employing indirectly heated thermistors.

Fig. 2Lis an alternative to Figure 1 employing directly heated thermistors.

Fig.3 is a schematic circuit diagram of a generator of waves more of a rectangular type employing directly heated thermistors; and

Fig. a shows wave-form diagrams for explain-' ing the operation ofFigure 3.

In this specification it is to be understood thatall resistances not referred to as thermistors are of the usual type in which the resistance value does not depend appreciably on the current through the resistance.

Fig. 1 shows an arrangement for producing wavesof a saw-tooth type. A is a differential relay or like type of switch having two equal winding 1.01 and 202. The relay is provided with two sets of change-over contacts a1 and a2 operated by currents in the windings an and 102. 'The relay should preferably be designed so that a current in winding 1171 in the direction of the arrow closes the upper contact of a1 and the lower contact of a2, while current in the direction... shown in the winding Z02 closes the lower contact of a1 and theupper contact of (12. .If currents are simultaneously present in both windings, then the contacts corresponding to the preponderating current will be closed. The relay should be adjusted so that a change-over does not occur until the difference between the two currents exceeds a predetermined value the relay maintaining its initial setting until the predetermined value of difference current is established. Thus once a change-over occurs the switched positioned is maintained until the difference between the currents in the two windings reaches the predetermined value, and is of a direction to cause change-over to the other position.

The relay windings L02 and un are respectively connected in series with the resistance elements R1 and R2 of two indirectly heated thermistors T1 and T2 and two resistances R5 and R6 to a battery or other suitable constant potential source E1. The heating coils T1 and r: of the thermistors T1 and T2 are connected across the battery E1 through the upper and lower contacts respectively of m. The output terminal I is connected to the junction of R5 and Re, and preferably also to earth, and the other output terminal 2 is connected to the moving spring contact of a2, whereby it may be switched alternately to R5 and Re.

It will be assumed that the upper contact of a1 and low contact of :12 are closed as shown when the battery is first switched on. The thermistors, being cold, will have a high resistance (assuming that they have a negative temperature coefiicient of resistance) so the voltage drop across R5 and Re Will be small. A low voltage will therefore be obtained at the output terminals. The currents in the two windings 101 and 102 bein at first equal there will be no force tending to change over the contacts a1 and a2.

The thermistor T1 now heats up on account of the current which flows through its heating coil 11, and the resistance R1 falls, thereby increasing the current in the circuit containing 7.02 and R5. The output voltage at terminals I and 2 rises at a rate determined by the characteristic of the thermistor, When the current through we exceeds that through Z01 by the predetermined amount, the contacts change over, and terminal 2 becomes connected to resistance Re and the voltage suddenly drops to the original low value. Thermistor T1 now cools and thermistor T2 heats up increasing the current in 2121 and raising the output voltage as before. At the same time the current through we decreases owing to the cooling of T1. The predetermined current difference will presently be obtained again in the .opposite direction, when the relay changes over and the rocess is repeated.

It will be evident that the output obtained at terminals I and 2 will be like the usual sawtooth wave obtained from a conventional relaxation oscillator, but the actual shape of the sloping portions of the waves will depend on the thermistor characteristics and the range of resistance variation employed. These are capable of considerable choice so that a considerable variety of wave forms is ossible. The frequency of the oscillations depends among other factors on the rate of heating of the thermistors.

Resistances (not shown) may be inserted in series or in parallel with the heating coils or with the resistance elements for assisting in the control of the frequency and wave form.

If the connections of the upper and lower contacts of :12 to R5 and Rs be reversed, it will be evident that an inverted saw-tooth type wave will be obtained at the output, that is, a wave in which the voltage is a maximum instead of a minimum after the flyback stroke. This wave will, however, have in general a different form depending on the cooling instead of on the hea ing of the thermistors.

If thermistors having a positive temperature coefiicient of resistance are used, the connection of the windings w and we should be reversed. An inverted wave will be obtained with the contacts az connected as shown in Figure 1. The usual type of wave can be obtained by reversing the connections of as.

A simpler arrangement for producing a similar type of wave employing two directly heated thermistors is shown in Fig. 2. A differential relay B with two windings generally similar to relay A of Fig. 1 has one set of change-over contacts b, the upper contact being closed as shown when the current in winding wi exceeds that in we. The windings 1121 and wz are connected in series respectively with the resistance elements R3 and R4 of the directly heated termistors T3 and T4 to a battery or other source E2, the circuit being completed through a common resistance R1 and through the change-over contacts 17. The output terminals I and 2 are connected to the extremities of R7, and terminal I which is connected to the negative terminal of the battery should preferably be connected to earth,

If the battery be supposed to be switched on with the upper contact 11 closed as shown, the thermistors being assumed to have a negative temperature coeflicient of resistance, there will be no current in winding tor, and a small current in 102 since the thermistor T4 is cold and will therefore have a high resistance. T4 starts to heat up and its resistance falls, thereby increasing the current in winding 202 until it becomes sufficient to change over the contacts I). The current in 2122 now becomes zero, and that in mi begins to increase due to the heating up of thermistor T3. In the meanwhile thermistor T4 cools and its resistance rises to a high value. When the current in U)! has reached the operating value, th relay changes over again and the process is repeated indefinitely.

During the period when T4 is heating up the potential across R7 (which is obtained at the output terminals I and 2) increases steadily from a low value until the relay changes over, when this potential suddenly falls to the original low value which applies when the thermistor T3 is cool. It then rises as before until the relay changes over again so that a saw-tooth type of wave is obtained as before.

The frequency of the waves depends chiefly on the rate of heating of the thermistors and the form of the sloping portions of the waves wi l, as before, depend on the shape of the thermistor characteristics. It will be evident that other resistances (not shown) may be included in the circuit if desired for assisting in the control of the operation.

If the thermistors have a positive temperature coeflicient a similar result will be obtained if the circuit of Fig. 2 is modified by connecting the thermistors in parallel instead of in series with the correspondin relay windings. In this case each thermistor has a low resistance when cold and will reduce the curent through the corresponding windin beow the operating value until it has had time to heat up. It will be evident that the operation will be substantially the same as other resistances elsewhere in the circuit. Reactive elements or additional thermistors may also be used in this or in either of the other circuits which have been described, for the purpose of modifying the output wave form.

What is claimed is:

1. A generator of cyclically varying currents, said currents varying from one value to a second value relatively smoothly over a long portion of a cycle due to thermistor action and returning quickly to its first value during a relatively short portion of a cycle due to switching action, said generator comprising at least one load resistor, a source of potential, two thermistors, connections for disposing said voltage source, load resistor and one thermistor in series, an output circuit connected across one of the resistive circuit elements, switching means for inter-changing thermistors, that thermistor in circuit being active and undergoing a temperature rise during its operation in circuit, and means controlled by current of said second value through the active thermistor for operating said switching means to inter-change thermistors.

2. The system of claim 1 wherein said switching means includes an electromagnetic relay having normal and off-normal contacts with the thermistors being connected through said contacts, and wherein each thermistor has a relay winding in series therewith, said two windings opposing each other.

3. A generator of cyclically varying currents, said currents varying from one value to a second value relatively smoothly over a long portion of a cycle due to thermistor action and returning quickly to its first value during a relatively short portion of a cycle due to switching action, said generator comprising a source of potential, two load resistors, two thermistors, connections for disposing said voltage source, one load resistor and one thermistor in series, an output circuit connected across one of the load resistors, switching means for inter-changing one thermistor and load resistor unit with the other thermistor and other load resistor, the thermistor in circuit being active and undergoing a temperature rise during its operation in circuit, and means controlled by the current of said second value through the active thermistor for operating said switching means.

4. The system of claim 3 wherein said switching means comprises a differential relay having normal and ofi-normal contacts for said two thermistors respectively.

5. The system of claim 3 wherein said differential relay has a pair of movable contacts having cooperating fixed contacts, and wherein one movable contact controls the selection of thermistor and wherein the other movable contact controls the selection of load resistor.

6. A generator of cyclically varying currents, said currents varying from one value to a second value relatively smoothly over a long portion of a cycle and returning quickly to its first value during a relatively short portion of a cycle, said generator comprising a pair of load resistors connected to have a common terminal, a thermistor connected to the free end of each load resistor respectively, a differential relay having opposed windings with a common terminal, a connection between the free end of a winding and thermistor respectively, a source of potential between the common winding terminal and load resistor terminal, two movable contacts controlled by said relay, each movable contact having fixed normal and off-normal cooperating contacts, a connection from the outer terminal of each load resistor to a respective fixed contact of said c0- operating contacts of one of said movable contacts, an output connection from said one movable contact, a common output terminal at the junction of said load resistors, a connection from the common winding terminal to the other movable contact, a heater for each thermistor, a connection from the common load resistor terminal to one terminal of each heater and a connection from the other terminal of each heater to the other respective fixed contact of said cooperating contacts of the other of said movable contacts.

7. A generator of cyclically varying currents comprising a switch having three banks of fixed contacts with a movable contact for each bank, said banks each having an equal even number of successive contacts, the movable contact for the first bank engaging two adjacent fixed contacts while the remaining movable contacts engage a single fixed contact at a time, a branch circuit for each fixed contact respectively of the first switch bank, each branch circuit being connected between its corresponding fixed switch contact and a common terminal, each branch including respectively a load resistor and thermistor in series, a source of potential connected between said common terminal and ground, a connection from the movable contact of the first switch bank to ground, connections from alternate fixed contacts of the second switch bank to ground, a connection for an output circuit to the movable contact of said second switch bank. connections from the remaining fixed contacts of the second switch bank to alternate fixed contacts of the third switch bank, a connection from each fixed contact of the third switch bank respectively to the junction between the load resistor and thermistor in a corresponding one of said branches, the connections to the three switch banks being so made that as the movable contacts progress in the same direction one of the two branches connected through the first switch bank will also be connected through the third switch bank, the entire switching system being so aligned that in any switch position two thermistor branches are energized with one of said branches being connected through the third switch bank and having the same active thermistor branch connected to the output circuit either in that switch position or in the adiacent switch position, every other switch position giving a no voltage output at the second bank, means responsive to a predetermined intensity of current for advancing said switch banks through successive single switching steps, said last-named means being connected between ground and the movable contact of the third switch bank.

PRAFULLA KUMAR CHATTERJEA. LESLIE WILFRED HOUGHTON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

